International patent application with publication No. WO 00/17488 discloses a system for drilling and logging of a wellbore formed in an earth formation, wherein a logging tool can be lowered in the wellbore from inside a drill string through a well drilling bit at the lower end of the drill string.
The known system comprises a drill bit including a bit body provided with a passageway for the logging tool, and a closure element for the passageway in the form of an insert section at the bit face. The bit body is attachable to a tubular drill string at a drill-string side of the bit body, and the passageway extends during normal operation in a well from an opening at the drill-string side to the well exterior of the bit body. The closure element comprises a bit-connecting means in the form of a primary latching device for selectively connecting the closure element to the bit body, so as to selectively close the passageway.
The latching device can be manipulated by an auxiliary tool that forms the downstream part of a logging tool string.
In the present specification and in the claims the terms upstream, upper and downstream, lower are used in relation to the lowering of a tool into a borehole, so that upstream, upper is closer to the surface than downstream, lower.
The logging tool string of the known system is arranged so that it can pass from the attached drill string through the opening of the bit body at the drill-string side, along the passageway so that it can reach the closure element, when the closure element is connected to the bit body. The auxiliary tool comprises a tool-connecting means in the form of a secondary latching device for selectively connecting the auxiliary tool to the closure element, so that the closure element is not lost in the wellbore when it is disconnected from the bit body.
The well drilling bit of the known system can be used for drilling operation, when the closure element is connected to the bit body. When it is desired to log the formation, drilling operation is stopped, and the logging tool string with the auxiliary tool at its lower end is lowered through the drill string into the passageway. The tool-connecting means (secondary latching mechanism) is connected to the closure element, and, simultaneously, the bit-connecting means (primary latching mechanism) is operated so as to release the closure element from the bit body. Then, the logging tool with the closure element attached to its lower end can be lowered into the wellbore ahead of the well drilling bit from where logging can be performed. After logging has been completed, the logging tool string can be pulled back into the drill string, so that the closure element is re-connected to the bit body and the auxiliary tool is simultaneously disconnected from the closure element.
International patent applications with publication Numbers WO03/004825, WO03/010410, WO2004/046505, WO2004/063522 disclose other embodiments of systems and methods for performing an operation in a wellbore ahead of a drill bit, wherein a tool is passed through a passageway in the bit body, connected to the closure element, and passed further to an external position in the borehole ahead of the bit body with the closure element connected to the lower end of the tool, at which external position the tool can be used to perform the operation.
While it is generally desirable to be able to re-connect to closure element to the drill bit after the operation in the borehole has been completed, it has been found that for some operations it is undesirable to have the closure element attached to the lower end of the tool.
It is an object of the present invention to provide a drill bit assembly that allows a tool to pass through the drill bit into the wellbore ahead, without having to connect to the closure element at the lower end of the tool.
It is a further object to provide a method for performing an operation in a wellbore wherein a tool is passed through a drill bit without having to connect the closure element at its lower end.